In recent years, standardization has been advanced at the IEEE (Institute of Electrical and Electronics Engineers) with respect to a wireless communication technology called WiMAX (Worldwide Interoperability for Microwave Access). WiMAX defines, as its standards, IEEE 802.16d intended for fixed subscriber stations and IEEE 802.16e intended for mobile subscriber stations. Also, the next generation of the IEEE 802.16e standard is currently in the process of standardization.
The next generation of the IEEE 802.16e standard is defined as specifications that support, as requirements, multicarrier applications using a plurality of frequency bands to provide wireless communication services. In multicarrier applications, the multiple frequency bands used are sometimes discontinuous in terms of frequency. Where a wireless communication system is operated using discontinuous frequency bands, a plurality of cells with different cell radii are often formed because the cell radius is dependent on the frequency band used. The cell denotes the range within which the radio waves from a base station reach.
FIG. 21 exemplifies the formation of cells. As illustrated in FIG. 21, a base station 101 radiates radio waves of a frequency band Fa over an area of a cell Fa1. A base station 102 radiates radio waves of the frequency band Fa over an area of a cell Fa2, and also radiates radio waves of a frequency band Fb over an area of a cell Fb1. A base station 103 radiates radio waves of the frequency band Fa over an area of a cell Fa3. The frequency bands Fa and Fb bear a frequency relationship of Fa>Fb.
The lower the radio frequency, the more the radio waves diffract, exhibiting higher NLOS (Non Line of Sight) transmission characteristics. Accordingly, compared with a cell of higher frequency, a cell of lower frequency can be increased in cell radius. In the example illustrated in FIG. 21, therefore, the cell Fb1 of the frequency band Fb lower than the frequency band Fa has a greater cell radius than the cells Fa1 to Fa3. Also, in the example of FIG. 21, since the cell Fb1 of the base station 102 covers the areas of the base stations 101 and 103, neither of the base stations 101 and 103 forms a cell of the frequency band Fb.
In FIG. 21, the lines extending from the respective base stations 101 to 103 and each having a trifurcated end symbolize the antennas of the base stations 101 to 103, respectively, and indicate that the cells Fa1 to Fa3 and Fb1 are formed around the base stations 101 to 103 with the base stations as their center. Also, although in FIG. 21 the cells Fa1 to Fa3 and the cell Fb1 are illustrated in the upper and lower parts, respectively, in order to indicate the difference between the frequency bands, the cells Fa1 to Fa3 are in fact accommodated in the cell Fb1.
FIG. 21 also illustrates a mobile device 111 for communicating wirelessly with the base stations 101 to 103. The mobile device 111 has two wireless units (in FIG. 21, indicated at RF1 and RF2) for communicating wirelessly in the respective frequency bands Fa and Fb. The wireless unit RF1 communicates wirelessly in the frequency band Fa, and the wireless unit RF2 communicates wirelessly in the frequency band Fb.
The lines extending from the wireless units RF1 and RF2 of the mobile device 111 indicate that the wireless units RF1 and RF2 are communicating wirelessly with the base stations 101 to 103 having the cells to which the respective lines are connected. Thus, the example of FIG. 21 indicates that when the mobile device 111 exists at a location A, the wireless unit RF1 communicates wirelessly with the base station 102 within the cell Fa2 of the frequency band Fa while the wireless unit RF2 communicates wirelessly with the base station 102 within the cell Fb1 of the frequency band Fb.
In the next generation of the IEEE 802.16e standard, a base station allocates, to mobile devices, respective identifiers for controlling the wireless communication. Such identifiers include an identifier uniquely allocated to each mobile device belonging to the base station (located within the coverage of the base station) and an identifier allocated so as to be unique within the mobile device. The former identifier, which is unique within the base station, is allocated so that a plurality of mobiles devices located within the coverage of the base station may have respective different identifiers. On the other hand, the latter identifier has only to be unique within the mobile device and may be identical with those allocated to other mobile devices located within the coverage of the same base station.
For example, when the mobile device 111 exists at the location A illustrated in FIG. 21, the wireless units RF1 and RF2 both belong to the base station 102. In this case, the base station 102 allocates the mobile device 111 an identifier that is unique within the base station 102, as well as an identifier that is unique within the mobile device 111.
Meanwhile, a wireless communication system has been disclosed which is configured to simultaneously provide communication services by using a plurality of discontinuous frequency bands (see International Publication Pamphlet No. WO2006/088082, for example). Also, there has been disclosed a wireless LAN system which has an increased number of channels and which can be used in both indoor and outdoor environments (see Japanese Laid-open Patent Publication No. 2003-101506, for example). Further, a communication system has been disclosed in which optimum parameter setting and multiuser scheduling are performed so as to follow a difference between the requested QoS and the variation characteristics of individual frequency bands (see Japanese Laid-open Patent Publication No. 2006-94005, for example).
If the target base station to which either of the wireless units of the mobile device is connected changes due to movement of the mobile device, however, the identifier unique within the base station occasionally needs to be reallocated even if the target base station to which the other wireless unit is connected remains unchanged, giving rise to the problem that the wireless communication is adversely affected.
Let it be assumed, for example, that in FIG. 21 the mobile device 111 exists at the location A and belongs to the base station 102 and that the base station 102 has allocated the mobile device 111 the identifier “1”, for example, which is unique within the base station 102.
Suppose that the mobile device 111 moves from the location A to a location B. In this case, the wireless unit RF2 of the mobile device 111 remains connected to the same base station 102 (cell Fb1) but the wireless unit RF1 connects to the base station 103 (cell Fa3), with the result that the mobile device 111 is connected to the base stations 102 and 103. If the identifier “1” is not used in the base station 103, the mobile device 111 can be continuously allocated the same identifier “1”.
On the other hand, if the identifier “1” is already used in the base station 103, the base stations 102 and 103 negotiate with each other so that a common identifier not used by either of the base stations may be reallocated to the mobile device 111. For example, if the identifier “2” is not used by either of the base stations 102 and 103, the mobile device 111 is reallocated the identifier “2”.
Where the wireless units RF1 and RF2 have come to belong to the different base stations 102 and 103, a common identifier is allocated to the mobile device 111 in order to allow the base stations 102 and 103 to control the mobile device 111 so that a single common MAC (Medium Access Control) entity encompassing the multiple wireless units of the mobile device 111 may be regarded as a single mobile device. If, for example, the mobile device 111 has different identifiers allocated by the different base stations 102 and 103, then it can hardly be said that the mobile device 111 is controlled by the base stations 102 and 103 so that the MAC entities thereof may be regarded as a single mobile device.
However, when the reallocation of the identifier is necessitated by the movement of the mobile device 111, the identifier is changed even if the target base station to which the wireless unit RF2 is connected remains unchanged. Such change of the identifier causes, for example, interruption of packets transmitted over the communication connection via the base station 102 or requires an additional authentication process, exerting an adverse influence on the wireless communication of the wireless unit RF2.